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Quantum States of Neutrons in the Gravitational Field

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Quantum States of Neutrons in the Gravitational Field Quantum States of Neutrons in the Gravitational Field Claude Krantz January 2006 i Quantum States of Neutrons in the Gravitational Field: We present an ex- periment performed between February and April 2005 at the Institut Laue Langevin (ILL) in Grenoble, France. Using neutron detectors of very high spatial resolu- tion, we measured the height distribution of ultracold neutrons bouncing above a reflecting glass surface under the e®ect of gravity. Within the framework of quan- tum mechanics this distribution is equivalent to the absolute square of the neutron's wavefunction in position space. In a detailed theoretical analysis, we show that the measurement is in good agreement with the quantum mechanical expectation for a bound state in the Earth's gravitational potential. We further demonstrate that the results can neither be explained within the framework of a classical calculation nor under the assumption that gravitational e®ects can be neglected. Thus we conclude that the measurement manifests strong evidence for quantisation of motion in the gravitational ¯eld as is expected from quantum mechanics and as has already been observed in another type of measurement using the same experimental setup. Finally, we study the possibilities of using the mentioned position resolving mea- surement in order to derive upper limits for additional, short-ranged forces that would cause deviations from Newtonian gravity at lenght scales below one millime- ter. QuantenzustÄandevon Neutronen im Gravitationsfeld Es wird ein Experi- ment vorgestellt, das von Februar bis April 2005 am Institut Laue Langevin (ILL) in Grenoble, Frankreich, durchgefÄuhrtwurde. Dabei wurde mit Hilfe von Neutro- nendetektoren mit sehr hoher Ortsaufl¨osungdie HÄohenverteilung ultrakalter Neu- tronen gemessen, die unter dem Einfluss der Schwerkraft Äuber einer reflektierenden Glasoberfl¨ache hÄupfen. In der Quantenmechanik entspricht diese HÄohenverteilung dem Betragsquadrat der Ortswellenfunktion eines Neutrons. Eine genaue Analyse der Messdaten ergibt, dass sich diese in guter Uberein-Ä stimmung mit der quantenmechanischen Erwartung fÄureinen gravitativ gebunde- nen Zustand be¯nden. Es wird gezeigt, dass die Messergebnisse weder rein klassisch, noch unter VernachlÄassigungdes Schwerepotentials beschreiben werden kÄonnen.Aus dieser Tatsache wird geschlossen, dass das Experiment starke Hinweise auf eine Quantisierung der Bewegung im Gravitationsfeld liefert, wie sie von der Quanten- mechanik vorhergesagt wird und in systematisch anderen Messungen mit dem selben experimentellen Aufbau bereits nachgewiesen wurde. Schlie¼lich wird untersucht, inwiefern aus besagten ortsaufl¨osendenMessungen Obergrenzen fÄurzusÄatzliche, kurzreichweitige KrÄafteabgeleitet werden kÄonnen.Sol- che KrÄaftewÄurden{ bei AbstÄandenunterhalb eines Millimeters { Abweichungen vom Newton'schen Gravitationsgesetz bewirken. ii Contents Introduction 1 1 Ultracold Neutrons and Gravity 3 1.1 The Neutron . 3 1.1.1 Fundamental Interactions . 3 1.1.2 Ultracold Neutron Production . 5 1.1.3 Interaction of Ultracold Neutrons with Surfaces . 8 1.2 Gravity and Quantum Mechanics . 10 1.2.1 The SchrÄodingerEquation . 10 1.2.2 Airy Functions . 11 1.2.3 The WKB Approximation . 13 1.2.4 Remarks . 14 2 The Experiment 17 2.1 Overview of the Installation . 17 2.2 The Waveguide . 19 2.3 Integral Flux Measurements . 22 2.4 Position Sensitive Measurements . 23 2.4.1 The CR39 Nuclear Track Detector . 24 iii iv CONTENTS 2.4.2 The Measurements . 26 2.4.3 The Data Extraction Process . 26 2.4.4 Data Corrections . 29 3 Quantum Mechanical Analysis 33 3.1 Observables of the Measurement . 33 3.1.1 Quantum Mechanical Position Measurement . 34 3.1.2 Time Dependence . 35 3.2 The Quantum Mechanical Waveguide . 38 3.2.1 Eigenstates . 38 3.2.2 Transitions within the Waveguide . 43 3.2.3 The Scatterer . 45 3.3 A First Attempt to Fit the Experimental Data . 47 3.3.1 The Fit Function . 48 3.3.2 Fit Results . 49 3.3.3 Groundstate Suppression Theories . 51 3.4 The Starting Population . 52 3.4.1 Transition into the Waveguide . 52 3.4.2 The Collimating System . 55 3.4.3 The Energy Spectrum . 58 3.4.4 Starting Populations . 59 3.5 Fit to the Experimental Data . 60 3.5.1 The 50-¹m Measurement (Detector 7) . 61 3.5.2 The 25-¹m Measurement (Detector 6) . 63 CONTENTS v 4 Alternative Interpretations 65 4.1 Classical View of the Experiment . 65 4.1.1 Classical Simulation of the Waveguide . 66 4.1.2 Classical Fit to the 50-¹m Measurement . 69 4.2 The Gravityless View . 73 4.2.1 The Waveguide without Gravity . 73 4.2.2 Gravityless Fit to the 50-¹m Measurement . 77 5 Gravity at Small Length Scales 83 5.1 Deviations from Newtonian Gravity . 83 5.2 A Model of the Experiment including Fifth Forces . 86 5.2.1 Fitting the Experimental Data . 87 5.2.2 Discussion of the Fit Results . 89 Summary 93 Bibliography 95 vi CONTENTS Introduction What is gravity? In many respects this may be one of the most important questions ever asked. Answers to it have always been milestones on the way from ancient natural philosophy to modern science. Every physicist knows the legendary free fall experiments of Galileo Galilei at the dawn of empiricism, forshadowing Newton's later formalism of the attraction of masses, itself the very starting point of modern physics and science as a whole. Since then, our comprehension of nature has changed. And paradoxally, the ques- tion that triggered it all is still one of the most obscure. The Standard Models of Cosmology and Particle Physics describe our world up to its largest and down to its smallest scales. Doubtlessly, both of these theories deserve being considered among the grandest cultural achievements in the history of mankind. Yet, they stand sepa- rate. The presently adopted view of gravity is provided by Einstein's General Theory of Relativity and has been unchallenged for almost a century. During this time, par- ticle physics has ¯rmly established that the three non-gravitational interactions are governed by the principles of quantum physics, which are inherently incompatible to the classical ¯eld concept of General Relativity. The latter is therefore expected to, sooner or later, have to be replaced by a quantised theory of gravity. Since 1999, an experiment at the Institut Laue Langevin in France is redoing Galilei's free fall experiments using as probe mass an elementary particle: the neu- tron. Although earlier attempts using atoms have been undertaken, this experiment is the ¯rst to have detected quantum states in the gravitational ¯eld by observing, at very high spatial resolutions, the motion of ultracold neutrons under the e®ect of gravity [Nesv02]. Its results may be considered as a ¯rst timid step on the way to bridge the gap separating gravitation and quantum physics. This diploma thesis presents a further measurement performed in 2005 using the mentioned experiment. By means of position resolving detectors, we directly imaged the height distribution of neutrons bouncing, under the e®ect of gravity, a few tens of micrometers above a reflecting surface. Within the framework of quantum mechanics, this distribution is equivalent to the absolute square of the neutron's wavefunction in position space. 1 2 INTRODUCTION In a ¯rst chapter of this text, we develop the basic concepts of quantised motion of an elementary particle in the ¯eld of gravity as they can be derived from quantum mechanics. Additionally, we provide the fundamentals of neutron physics as far as they are relevant to our experiment. Special attention is payed to the unique properties of ultracold neutrons, without which the mentioned measurement would be impossible to realise. The latter is throughoutly described in chapter 2, where we give an overview of the experimental techniques put into operation and discuss the data taking processes. The main focus of this thesis lies on chapter 3, where we further develop the re- sults from the ¯rst chapter into a detailed theoretical description of the experiment and the measurement process. Within the framework of quantum mechanics, this yields a prediction of the density distribution of low-energetic neutrons bouncing under the e®ect of gravity. In order to rule out possible misinterpretations of the data, we subsequently oppose this quantum mechanical expectation to two alterna- tive ones described in chapter 4: The ¯rst neglects the e®ects of quantum mechanics, the second those of gravity. All three models are compared to the neutron height distributions that have actually been measured. Over the last years, a number of theories have arisen which, in an e®ort to derive a quantum description of gravity, predict deviations from the Newtonian gravitational potential at length scales below one millimeter [Ark98] [Ark99]. Thus, in a last chapter, we study possible upper limits on such deviations that could be obtained from the type of measurement we have performed. Chapter 1 Ultracold Neutrons and Gravity 1.1 The Neutron Over the last decades the neutron has has become a tool of ever increasing impor- tance to both the ¯elds of fundamental and applied physics. With a rest mass m = 939:485(51) MeV/c2, the neutron is the second-lightest member of the baryon octet. As such, it can in principle be used to probe all four of the fundamental interactions. While this is not astonishing in itself, there are a few notable properties that do make the neutron special among all baryons: It is electrically neutral, easy enough to produce and su±ciently long-lived to be a very practical probe particle. Additionnally, neutrons may be cooled down to kinetic energies of less than 1 meV, which are then of the same order of magnitude than the potentials of their fundamental interactions. At the latter we will have a closer look in the following subsection. 1.1.1 Fundamental Interactions Neutrons are electrically neutral. In the valence quark model, the neutron consists of one up and two down quarks and therefore carries an electric charge equal to 0.
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